1. Field of the Invention
This invention relates to power lawn mowers and more particularly, improvements of power lawn mowers comprising a housing having a short cylindrical peripheral wall defining a lawn drop opening and an upper wall connected to an upper edge of the peripheral wall, a plurality of travelling wheels supporting the housing, an engine mounted on the housing upper wall and having a driving shaft projecting into the housing, and a cutter mounted on the driving shaft and rotatably driven within the peripheral wall.
2. Description of the Prior Art
When mowed lawn grass or cuttings are dropped onto a lawn surface and the size of the cuttings is large, the patch of lawn covered with the cuttings is poor in transmission of light and in permeability of air, resulting in fear that the growth of such lawn grass so covered with the cuttings is damaged. Thereupon, it is necessary to chop the mowed lawn grass into short pieces so as to drop such short pieces between blades of the lawn grass.
There is a known power lawn mower (see U.S. Pat. No. 4,158,279) in which in order to meet the requirement for short pieces, an inwardly projecting portion is provided over the entire lower edge of the peripheral wall, so that mowed lawn pieces are retained in a region within a radius of rotation of the cutter within the housing.
If the inwardly projecting portion is provided over the entire lower edge of the peripheral wall of the housing, as described above, the mowed lawn pieces are retained in an excessive amount in the housing, thereby causing an increased load on the cutter, resulting in the cutter failing to sufficiently fulfill its intrinsic mowing function and in deteriorated durability of the engine.
There is also a further problem that the lawn grass cannot be effectively mowed because a front portion of the inwardly protruding portion, in a direction of forward movement of the lawn mower, acts to delay the time for the lawn grass pushed down by the peripheral wall to be restored to a standing state.
In addition, in the power lawn mower of such prior art type, the cutter is rotated from the front, toward the rear, in the direction of forward cutting movement of the lawn mower at one side of a vertical plane parallel to the direction of such forward cutting movement of the lawn mower and bisecting the housing, but is rotated from the rear toward the front on the other side of such vertical plane.
When the lawn grass is mowed by such power lawn mower, the cutting modes of the cutter on one on the other sides of such vertical plane are different from each other and, correspondingly, areas of the lawn surface cut on the one side are different in condition from areas cut on the other side. Therefore, there is a need for a means for providing a uniform condition of the lawn surface over such two mowed areas. Such means is not found in prior art power lawn mowers.
Hence, the lawn surface mowed with the prior art power lawn mower has a mottled pattern and an inferior external appearance.
Further, in a lawn mowing operation by the lawn mower of such prior art type, a patch of lawn grass falling down by the travelling wheels, particularly, by the front wheels, lacks firmness and hence, is liable to be pushed back, outwardly of the peripheral wall, by the left and right sides of the peripheral wall, with the result that lawn grass portions left uncut are apt to be generated along a treading path of each of the front wheels. Means for preventing such lawn grass uncut is not provided in the prior art power lawn mower.
In such circumstances, the lawn surface cannot be mowed uniformly by a single lawn mowing operation and, therefore, the lawn mowing operation must be carried out once more along the treading path of each front wheel, resulting in inferior workability.